Many types of fluids have been used in the drilling of oil and gas wells. The selection of an oil-based drilling fluid, also known as oil-based mud, involves a careful balance of the both the good and bad characteristics of such fluids in a particular application, the type of well to be drilled and the characteristics of the oil or gas field in which the well is to be drilled. A surfactant capable of emulsifying incorporated water into the oil is an essential component of oil-based muds.
The primary benefits of selecting an oil-based drilling fluid include: superior hole stability, especially in shale formations; formation of a thinner filter cake than the filter cake achieved with a water based mud; excellent lubrication of the drilling string and downhole tools; penetration of salt beds without sloughing or enlargement of the hole as well as other benefits that should be known to one of skill in the art.
An especially beneficial property of oil-based muds is their excellent lubrication qualities. These lubrication properties permit the drilling of wells having a significant vertical deviation, as is typical of off-shore or deep water drilling operations or when a horizontal well is desired. In such highly deviated holes, torque and drag on the drill string are a significant problem because the drill pipe lies against the low side of the hole, and the risk of pipe sticking is high when water based muds are used. In contrast oil-based muds provide a thin, slick filter cake which helps to prevent pipe sticking and thus the use of the oil based mud can be justified.
Despite the many benefits of utilizing oil-based muds, they have disadvantages. In general the use of oil based drilling fluids and muds has high initial and operational costs. These costs can be significant depending on the depth of the hole to be drilled. However, often the higher costs can be justified if the oil based drilling fluid prevents the caving in or hole enlargement which can greatly increase drilling time and costs. Disposal of oil-coated cuttings is another primary concern, especially for off-shore or deep-water drilling operations. In these latter cases, the cuttings must be either washed clean of the oil with a detergent solution which also must be disposed of, or the cuttings must be shipped back to shore for disposal in an environmentally safe manner. Another consideration that must be taken into account is the local governmental regulations that may restrict the use of oil based drilling fluids and muds for environmental reasons.
Oil-based muds contain some water, either formed in the formulation of the drilling fluid itself, or residual water in the hole, or intentionally added water to affect the properties of the drilling fluid or mud. In such water-in-oil type emulsions, also know as invert emulsions, a emulsifier is utilized that will stabilize the emulsion. In general, the invert emulsion may contain both water soluble and oil soluble emulsifying agents. Typical examples of such emulsifiers include polyvalent metal soaps, fatty acids and fatty acid soaps, and other similar suitable compounds that should be known to one of skill in the art. The use of traditional emulsifiers and surfactants in invert drilling fluid systems can complicate the clean up process in open hole completion operations. Fluids using traditional surfactant and emulsifier materials may require the use of solvents and other surfactant washes to penetrate the filter cake and reverse the wetability of the filter cake particles. That is to say the washing with detergents should convert the oil-wet solids of the filter cake into water-wet solids. Water-wet solids in the filter cake are necessary so that the subsequent acid wash can attack the particles of the mud cake and destroy or remove them prior to production. The productivity of a well is somewhat dependent on effectively and efficiently removing the filter cake while minimizing the potential of water blocking, plugging or otherwise damaging the natural flow channels of the formation. The problems of efficient well clean-up, stimulation, and completion are a significant issue in all wells, and especially in open-hole horizontal well completions.
A typical horizontal well completion process includes one or more of the following: drilling the horizontal section utilizing an oil-based drilling fluid; smoothing directional corrections with a hole opener, displacing the open hole section with an unused drill-in fluid to minimize solids exposed to the completion assembly; running the completion assembly in to the horizontal well; displacing the drill-in fluid with a completion brine; washing the filter cake with solvents and surfactants to remove or wash away the oil-based drilling fluid; destroying the filter cake with an acid soak; and, commencing production. Extension of the time required to clean up the open hole well can result in wellbore instability and possible collapse. The collapse of a well is generally considered a bad occurrence because the well will then have to be redrilled or opened up if production from the formation is to occur. Thus the stability of the open-hole well limits the number of steps performed before commencing production. Thus there is a tradeoff between increased production due to a fully cleaned-up well bore and the potential of well collapse due to instability.
In view of the above there exists an unmet need for an oil-based drilling fluid or mud emulsion that can easily be broken in the presence of the acid soak solution. Such a fluid would allow a decrease in the number of steps involved in removing the filter cake and cleaning up the well which minimizes the risk of well collapse. In addition such a fluid would allow for a more thorough and complete cleaning up of the well thus increasing the production of the well.